1. Field of the Invention
The present invention relates to a method for controlling combustion in an internal combustion engine, particularly a direct-injection engine, and an engine using such a method such as Diesel engines with burned-gas recirculation, and particularly engines of this type that can operate in two combustion modes.
2. Description of the Prior Art
A traditional combustion mode injects the fuel around the Combustion Top Dead Center with diffusion combustion preferably being used at high loads, while another combustion mode, known as homogenous combustion, is used at low and medium loads.
In this latter combustion mode, it is known that the fuel coming from an injector can be mixed with the gaseous fluid or fluids admitted into the combustion chamber of this engine, such as air or a mixture of air and recirculated exhaust gas, in order to obtain a homogenous air-fuel mixture before combustion begins.
This is known, particularly for Diesel engines, by the general term Homogenous Charge Compression Ignition (abbreviated HCCI) and the assignee has developed such a combustion mode using a fuel injector with a small crank angle to avoid wetting the cylinder walls with the injected fuel. This not only prevents degradation of the lubricant present on this wall, but also prevents increases in pollutant emissions and reduction in engine performance, particularly in terms of fuel consumption. Development of this combustion mode is described more precisely in French Patents 2,818,324 and 2,818,325 by the assignee offering such a process used under the name NADI™.
Moreover, at low and medium loads, this homogeneous combustion mode generates only low flame temperatures upon combustion of the air-fuel mixture in the combustion chamber, which considerably cuts down on nitrogen oxide (Nox) and particle emissions while preserving engine performance.
To favor such a combustion mode, it is desirable to introduce not only intake air but large quantities of burned gases from the exhaust and to associate this burned gas recirculation with specific fuel injection strategies. The means most commonly used to achieve such recirculation is to send some of the exhaust gases to the engine intake through an external circuit known as EGR (Exhaust Gas Recirculation).
On the other hand, as already mentioned, such engines are also designed to operate by traditional combustion, which requires more moderate burned-gas levels and different injection strategies from those used in homogeneous combustion.
The problem encountered with this type of engine operating in two combustion modes resides in the fact that it is difficult to ensure rapid, precise control of the mass of air and/or burned gases admitted into the cylinder as a function of driver demand, because the dynamics of the air loop (burned air and/or gas) are relatively slow, about a few seconds, particularly by comparison to the dynamics of the fuel loop which reacts on the order of the engine combustion cycle. Hence, in a transient operating phase, the air and/or burned-gas settings may be unsuitable for the fuel settings, which is not conducive to optimal combustion in terms of pollutant emissions, combustion noise, or fuel consumption.